Mitra chat input bar: drop maxLines:3 attempt + nudge alignment down

Follow-up to 92da8b2. With `textAlignVertical: center` + `isDense:true`,
the TextField was centering the line-box baseline on the parent
midline — but Latin lowercase glyphs sit at ~75% of line height,
leaving descender space empty below and the optical center of text
visibly above the pill midline.

Fix: `textAlignVertical: TextAlignVertical(y: 0.4)` shifts the
baseline down to align Latin x-height optical centers with the pill
midline. Also added explicit `alignment: Alignment.center` on the
Container so the field's small intrinsic line-box positions on the
midline rather than docking to the top.

Verified on emulator-5556 driving the typed "halo" through the chat
input — text body now sits on the visual midline of the 44dp pill.

The horizontal underline below typed text is Gboard's composing-
region indicator (Android IME behavior), not a TextField underline,
and will go away once the user commits the word with space/send.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mitra_app/lib/features/chat/screens/mitra_chat_screen.dart

@@ -742,6 +742,11 @@ class _MitraChatBodyContentState extends ConsumerState<_MitraChatBodyContent> {
               Expanded(
                 child: Container(
                   height: 44,
+                  // alignment.center positions the TextField on the vertical
+                  // midline of the 44dp container. Without this, isDense's
+                  // small intrinsic height docks to the top of the parent
+                  // (no implicit vertical centering for Container children).
+                  alignment: Alignment.center,
                   padding: const EdgeInsets.symmetric(horizontal: 16),
                   decoration: const BoxDecoration(
                     // White bg so the pill stands out against the cream page
@@ -756,29 +761,37 @@ class _MitraChatBodyContentState extends ConsumerState<_MitraChatBodyContent> {
                       BorderSide(color: HaloTokens.border),
                     ),
                   ),
-                  // Center wrapper makes the TextField sit on the vertical
+                  // Vertical centering recipe: Container.alignment.center
-                  // midline of the 44dp pill. `textAlignVertical: center`
+                  // positions the TextField on the midline; textAlignVertical
-                  // + `isCollapsed: true` alone don't center against the
+                  // with a positive y (~0.4) nudges the baseline down so the
-                  // parent height — they only center within the field's
+                  // optical center of Latin lowercase glyphs lines up with
-                  // own intrinsic line-box, which then docks to the top of
+                  // the pill midline, not just the baseline. With y=0 (true
-                  // the parent. Wrapping in Center delegates the vertical
+                  // center), the field's line-box centers but Latin text
-                  // alignment to the container's stack.
+                  // bodies sit in the upper half because the baseline is at
-                  child: Center(
+                  // ~75% of line height — leaving descender space empty
+                  // below. y=0.4 shifts down to match the visual midline.
                   child: TextField(
                     controller: widget.messageController,
                     onChanged: widget.onTextChanged,
                     textInputAction: TextInputAction.send,
                     onSubmitted: (_) => widget.onSend(),
-                      style: const TextStyle(fontSize: 13.5, color: HaloTokens.ink),
+                    maxLines: 1,
+                    textAlignVertical: const TextAlignVertical(y: 0.4),
+                    style: const TextStyle(
+                      fontSize: 13.5,
+                      color: HaloTokens.ink,
+                    ),
                     decoration: const InputDecoration(
                       hintText: 'ketik balasan...',
                       hintStyle: TextStyle(color: HaloTokens.inkMuted, fontSize: 13.5),
-                        isCollapsed: true,
+                      isDense: true,
+                      contentPadding: EdgeInsets.zero,
                       border: InputBorder.none,
                       enabledBorder: InputBorder.none,
                       focusedBorder: InputBorder.none,
                       disabledBorder: InputBorder.none,
-                      ),
+                      errorBorder: InputBorder.none,
+                      focusedErrorBorder: InputBorder.none,
                     ),
                   ),
                 ),